Concrete mixing machines may be divided into two general classes, batch mixers and continuous mixers. In the former, sufficient materials are proportioned to make a convenient sized batch for the mixer. They are then charged into the machine at once, given a certain amount of mixing, and then discharged at once. In the continuous mixers the materials are dumped on a platform, and after being properly proportioned, are delivered gradually to the mixer, and if fed uniformly, the concrete is discharged continuously by the machine. In the latter method care must be taken to feed the cement, sand and stone together and at a uniform rate. If one man shovels cement, two men shovel sand and four men handle the stone, and the cement man stops to fill his pipe, there is likely to be a poor streak of concrete. It is therefore desirable in feeding a continuous mixer to spread the measured quantity of stone on the platform, and on top of this place the weighed quantities of sand and cement. Then if each shoveler gets his shovel blade under the whole mass, he will have some of each ingredient.
1 Described by C. M. Saville, M. Am. Soc. C. E., Engineering News, Mar. 13, 1902.
325. There are many styles of concrete mixers of both classes on the market. One of the oldest, as well as one of the best, is the cubical box mixer which consists of a box four or five feet on a side, supported by trunnions at opposite corners, and made to revolve about this axis. A hinged door is provided near one corner of the box by which the latter is charged and emptied. The dry materials may be first charged and mixed and the water added later, either through the door or through a perforated pipe in the axis, or the water may be added with the dry materials; after from ten to thirty revolutions of the box, the mixed concrete is discharged into a skip or on a car, to be conveyed to the place of deposition.
The great merit of this mixer is that the materials are thrown back and forth from one side of the cube to another and a thorough commingling results. The chief disadvantage is the difference in elevation between the receiving hopper and point of delivery, making it necessary to elevate the materials; one other defect is that the batch is not in view while being mixed, so that the amount of water cannot be regulated according to slight variations that may occur in the moisture of the sand and stone when charged.
326. To obviate this latter difficulty as well as to facilitate to some extent the charging and dumping of the batch, a form of box mixer is made in which the corners of the box in the axis of revolution are truncated, and the trunnions are replaced by collars which support the box, and through which the materials may be fed and discharged. The collars are supported in a tilting cradle which permits the delivery end to be depressed after the batch is mixed. This style of mixer with truncated corners and tilting cradle is known as the Chicago Improved Cube.1
Mixers working on the same principle are sometimes made in other forms than the cube. One of these is the cylindrical mixer, which is made of boiler plate and may be four or five feet in diameter and five or six feet long. This is rotated about a diagonal axis. It is said to be more easily and cheaply made than the cubical mixer, and dumps more quickly and cleanly, while the Cost of operation is about the same, and the mixing is as satisfactorily done as in the cubical form.
327. The so-called " Dromedary Mixer " 2 is a batch mixer specially designed for use on street work. The mixing chamber is a cylindrical steel drum with closed ends, mounted between two wheels. It is hinged along an element of the cylinder so that it opens into two halves like a clam shell bucket, to discharge. A trap door is provided for filling. The cart is drawn by a horse, and the chamber may be thrown in or out of gear with the cart wheels. The cement and sand being first added and the trap door closed, the horse draws the cart to the stone pile. The stone and water are here added and the cart is drawn to the work; the concrete, mixed on the way, is dumped by the driver, who merely raises a lever which not only separates the two halves of the mixer, but throws it out of gear so that it stops revolving. The chamber may be thrown out of gear at any time without dumping if desired.
The Ransome Concrete Mixer 3 "consists of a hollow rotary dome, having upon the inner surface of its periphery directing guides or flanges, and hinged shelves, by means of which the materials are thrown together and perfectly commingled. A discharge chute, or spout, is arranged to deliver the material into the barrow or cart when properly mixed." The mixer is also provided with an automatic device for proportioning the materials, and a conveyor to carry them to the mixer. water is supplied to the mixer through a pipe with facilities for regulating the supply.
1 Municipal Engineering and Contracting Co., Railway Exch., Chicago, III.
2 Manufacture now discontinued.
3 Ransome Concrete Machinery Co., 11 Broadway, N. Y.
The Smith Mixer1 is a batch machine made of two truncated cones placed base to base, and provided on the interior with deflecting plates designed to throw the materials from one end of the mixer to the other as the machine is revolved. At the junction of the two cones, on the outer circumference, is a spur gear by which the chamber is actuated. The latter rests upon rollers in a swinging frame, so arranged that the machine may be tilted for dumping while the drum is revolving. In operating this mixer it has been found advantageous to charge the broken stone or gravel first, and give one or two revolutions before adding the cement and sand, as this cleans the mortar from the corners. This form seems to be particularly adapted for a portable machine. They may be had mounted on trucks, with or without an engine, as desired.